A variety of implantable medical devices (IMD's) exist that provide diagnostic or therapeutic capabilities. These IMD's include, for example, cardiac pacemakers, implantable cardioverters/defibrillators (ICD's), and various tissue, organ and nerve stimulators or sensors. IMD's typically include their components within a hermetically sealed enclosure referred to as a “can” or housing. In some IMD's, a connector header or connector block is attached to the housing and allows interconnection with one or more elongated electrical medical leads.
The header is typically molded from of a relatively hard, dielectric, non-conductive polymer having a thickness approximating the housing thickness. The header includes a mounting surface that conforms to and is mechanically affixed against a mating sidewall surface of the housing.
It has become common to provide a communication link between the hermetically enclosed electronic circuitry of the IMD and an external programmer or monitor or other external medical device (herein an EMD unless otherwise identified) in order to provide for downlink telemetry (DT) transmission of commands from the external device to the IMD and to allow for uplink telemetry (UT) transmission of stored information and/or sensed physiological parameters from the IMD to the EMD. As the technology has advanced, IMDs have become ever more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring increasing varieties of physiologic conditions and electrical signals which place ever increasing demands on the programming system. Conventionally, the communication link between the IMD and the EMD is by encoded RF transmissions between an IMD RF telemetry antenna and transceiver and an EMD RF telemetry antenna and transceiver.
The telemetry transmission system that evolved into current common use relies upon the generation of low amplitude magnetic fields by current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode and the sensing of currents induced by a closely spaced RF telemetry antenna in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In some products, the RF carrier frequency is set at 175 kHz, and the RF telemetry antenna is coiled wire wound about a ferrite core. The EMD is typically a programmer having a manually positioned programming head having an external RF telemetry antenna. Generally, the antenna is disposed within the hermitically sealed housing; however, the typically conductive housing adversely attenuates the radiated RF field and limits the data transfer distance between the programmer head and the IMD RF telemetry antennas to a few inches.
The above described telemetry system employing the 175 kHz carrier frequency limits the upper data transfer rate, depending on bandwidth and the prevailing signal-to-noise ratio. Using a ferrite core/wire coil, RF telemetry antenna results in: (1) a very low radiation efficiency because of feed impedance mismatch and ohmic losses; 2) a radiation intensity attenuated proportionally to at least the fourth power of distance (in contrast to other radiation systems which have radiation intensity attenuated proportionally to square of distance); and 3) good noise immunity because of the required close distance between and coupling of the receiver and transmitter RF telemetry antenna fields.
With these characteristics, the IMD is subcutaneously and preferably oriented with the RF telemetry antenna closest to the patient's skin. To ensure that the data transfer is reliable, the programming head and corresponding external antenna are positioned relatively close to the patient's skin.
It has been recognized that “far field” telemetry, or telemetry over distances of a few too many meters from an IMD would be desirable. Various attempts have been made to provide antennas with an IMD for facilitate far field telemetry. Many proposals have been advanced for eliminating the ferrite core, wire coil, RF telemetry antenna and substituting alternative telemetry transmission systems and schemes employing far higher carrier frequencies and more complex signal coding to enhance the reliability and safety of the telemetry transmissions while increasing the data rate and allowing telemetry transmission to take place over a matter of meters rather than inches. A wide variety of alternative IMD telemetry antennas mounted outside of the hermetically sealed housing have been proposed. These approaches are generally undesirable in that depending upon the option selected they require substantial modification of the housing and/or heading, require additional components added to the housing (e.g., dielectric shrouds about a portion of the housing), reduce the effectiveness of other components (e.g., reducing the surface area available for use as a can electrode), create a directional requirement (e.g., require that the IMD be oriented in a particular direction during implant for telemetry effectiveness), or finally that they add extraneous exposed components that are subject to harmful interaction in the biological environment or require additional considerations during implant (e.g., stub antennas extending outward from the device).
It remains desirable to provide a telemetry antenna for an IMD that eliminates drawbacks associated with the IMD telemetry antennas of the prior art. As will become apparent from the following, the present invention satisfies this need.